'Throughout history and in virtually all human societies marriage has always been the union of a man and a woman." So says the Coalition for Marriage, whose petition against same-sex unions in the UK has so far attracted 500,000 signatures. It's a familiar claim, and it is wrong. Dozens of societies, across many centuries, have recognised same-sex marriage. In a few cases, before the 14th century, it was even celebrated in church.

This is an example of a widespread phenomenon: myth-making by cultural conservatives about past relationships. Scarcely challenged, family values campaigners have been able to construct a history that is almost entirely false.

This theme was aggressively pursued by the church for some 1,500 years. In his classic book A World of Their Own Making, Professor John Gillis points out that until the Reformation, the state of holiness was not matrimony but lifelong chastity. There were no married saints in the early medieval church. Godly families in this world were established not by men and women, united in bestial matrimony, but by the holy orders, whose members were the brothers or brides of Christ. Like most monotheistic religions (which developed among nomadic peoples), Christianity placed little value on the home. A Christian's true home belonged to another realm, and until he reached it, through death, he was considered an exile from the family of God.

The Reformation preachers created a new ideal of social organisation – the godly household – but this bore little relationship to the nuclear family. By their mid-teens, often much earlier, Gillis tells us, "virtually all young people lived and worked in another dwelling for shorter or longer periods". Across much of Europe, the majority belonged – as servants, apprentices and labourers – to houses other than those of their biological parents. The poor, by and large, did not form households; they joined them.

The father of the house, who described and treated his charges as his children, typically was unrelated to most of them. Family, prior to the 19th century, meant everyone who lived in the house. What the Reformation sanctified was the proto-industrial labour force, working and sleeping under one roof.

The belief that sex outside marriage was rare in previous centuries is also unfounded. The majority, who were too poor to marry formally, Gillis writes, "could love as they liked as long as they were discreet about it". Before the 19th century, those who intended to marry began to sleep together as soon as they had made their spousals (declared their intentions). This practice was sanctioned on the grounds that it allowed couples to discover whether or not they were compatible. If they were not, they could break it off. Premarital pregnancy was common and often uncontroversial, as long as provision was made for the children.

The nuclear family, as idealised today, was an invention of the Victorians, but it bore little relationship to the family life we are told to emulate. Its development was driven by economic rather than spiritual needs, as the industrial revolution made manufacturing in the household unviable. Much as the Victorians might extol their families, "it was simply assumed that men would have their extramarital affairs and women would also find intimacy, even passion, outside marriage" (often with other women). Gillis links the 20th-century attempt to find intimacy and passion only within marriage, and the impossible expectations this raises, to the rise in the rate of divorce.

Children's lives were characteristically wretched: farmed out to wet nurses, sometimes put to work in factories and mines, beaten, neglected, often abandoned as infants. In his book A History of Childhood, Colin Heywood reports that "the scale of abandonment in certain towns was simply staggering", reaching one third or a half of all the children born in some European cities. Street gangs of feral youths caused as much moral panic in late 19th-century England as they do today.

Conservatives often hark back to the golden age of the 1950s. But in the 1950s, John Gillis shows, people of the same persuasion believed they had suffered a great moral decline since the early 20th century. In the early 20th century, people fetishised the family lives of the Victorians. The Victorians invented this nostalgia, looking back with longing to imagined family lives before the industrial revolution.

The conservatives' supposedly moral concerns turn out to be nothing but an example of the age-old custom of first idealising and then sanctifying one's own culture. The past they invoke is fabricated from their own anxieties and obsessions. It has nothing to offer us.